FIG. 6 is a block diagram of a conventional automatic volume controller disclosed in Japanese Patent Laid-Open No. 5-30588. A signal entered through an input terminal 1 is amplified in power with a first amplifier 2 and is reproduced through a loudspeaker 3. The gain of the first amplifier 2 is controlled by a control signal from a control microcomputer 13. A microphone 6 located near the loudspeaker 3 captures a reproduced sound 4 from the loudspeaker 3 and a surrounding noise 5. The output of the microphone 6 is amplified with a second amplifier 7. A phase inverter 8 and a compensation circuit 9 correct a level and phase of the output of the first amplifier 2 at every frequency so as to cancel a component from the loudspeaker 3 out of the output from the second amplifier. Outputs of the second amplifier 7 and the compensation circuit 9 are added to each other with an adder 10. The adder 10 extracts only the surrounding noise around the loudspeaker 3. The output of the adder 10 is smoothed to be a direct-current with an integrating circuit 11, is digitized by an analog-to-digital (A/D) converter 12, and is put into the control microcomputer 13.
When the noise is ignorable, the volume of the loudspeaker 3 is set at a predetermined level (initial volume). That is, the microcomputer 13 sets the gain of the first amplifier 2 based on only a volume-operation signal 14. The control signal from the microcomputer 13 at this moment is a reference control signal. The output signal of the A/D converter 12 at this moment is zero.
When the noise is generated, the microcomputer 13 calculates the ratio of the reference control signal to the output of the A/D converter 12. The ratio is further compared with a reference ratio which is a threshold for determining that the gain of the first amplifier 2 is raised. When the ratio is below the reference ratio, the initial volume is maintained. That is, it is determined that the noise is not too serious as to raise the volume. On the other hand, when the ratio exceeds the reference ratio, a portion exceeding the reference ratio is divided into predetermined ranges. Then, the microcomputer 13 outputs a control signal so as to raise the volume according to a specified range of the ranges. The control signal changes the gain of the first amplifier 2 to adjust the volume of the sound reproduced through the loudspeaker 3.
In a conventional automatic volume controller, when the ratio is less than the reference ratio, the microcomputer 13 does not output the control signal to raise the gain of the first amplifier 2, thus not controlling the volume despite the existing noise.
Even if the ratio exceeds the reference ratio, a control amount according to the range must be preliminarily determined. Further, the control signal changes the gain of the first amplifier 2 in steps, not continuously, thus having the loudspeaker 3 reproduce sound unnaturally.